1. Field of the Invention
The present invention relates to the vertical blanking interval (VBI) of TV signals, and more particularly to an apparatus and method for detecting VBI signals.
2. Description of the Prior Art
The vertical blanking interval (VBI) includes the scan lines of TV signals reserved for appending various kinds of digital information by TV system providers, so that the audience can obtain various information services such as consuming information, stock market information, program schedule, etc., besides watching standard TV programs. Various VBI specifications have been developed, such as Closed Caption (CC), Copy Generation Management System (CGMS), Wildscreen Signaling (WSS), Video Programming System (VPS), and Teletext. The TV system provider encodes and appends the information to be transmitted to the VBI interval of the TV signal at the transmitting side. Then, after the TV signal is received at the receiving side, the VBI signal is decoded to retrieve the original information.
FIG. 1 is a diagram showing a Teletext VBI signal contained in a scan line. As shown, the VBI signal comprises a horizontal synchronization (Hsync) signal, a color burst signal, a clock run-in signal and data. When a VBI decoder at the receiving side receives a TV signal, it detects whether there is a VBI signal contained in the VBI interval. In the prior art, a window with a fixed width is set in the VBI interval to detect whether the TV signal contains a clock run-in signal. If the TV signal contains the clock run-in signal, then a VBI signal is detected. Since the clock run-in signal may drift and deform due to the signal quality of various system providers and interference during the transmission process, etc., and different VBI signal rules are adopted by various system providers, the conventional VBI decoder applies a wider and fixed window to assure the detection of the clock run-in signal. However, the VBI interval contains various kinds of noises in addition to the VBI signal. The wider the window is, the more possible a false alarm occurs. Thus, it is easier to mistake the detected noise as the clock run-in signal, and the accuracy of the conventional VBI decoder is degraded.